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Solutions for laser cladding defects


Laser cladding means that the selected coating material is placed on the surface of the substrate to be cladding in different ways, and is irradiated with a thin layer of the surface of the substrate at the same time by laser irradiation. After rapid solidification, the degree of dilution is extremely low. The surface coating which is metallurgically combined with the substrate can significantly improve the abrasion resistance, corrosion resistance, heat resistance, oxidation resistance and electrical characteristics of the surface of the substrate, so as to achieve the purpose of surface modification or repair. The performance requirements have saved a lot of valuable elements. However, in actual application, we found that laser cladding products have defects. This article intends to analyze the formation of laser cladding defects and provide some suggestions.

First, the quality of laser cladding metallurgy combination: The degree of bond between the cladding layer and the substrate should theoretically be a dense, low-dilution, narrow interdiffusion band. To meet this requirement, in addition to the laser processing technology and the thickness of the cladding layer, it depends on the properties of the alloy material and the substrate material; good wettability and self-melting can obtain the ideal metallurgical combination. However, if the melting point difference between the cladding layer alloy and the base material is too large, a good metallurgical bond cannot be formed. The melting point of the cladding layer alloy is too high, the cladding layer has a small melting, the surface finish is reduced, and the substrate surface is overheated, which seriously contaminates the coating; on the contrary, the overheating of the coating causes the alloying elements to evaporate and the shrinkage rate increases, destroying the structure of the coating And performance. At the same time, the substrate is difficult to melt, the interfacial tension increases, and holes and inclusions are unavoidable between the coating and the substrate. In the process of laser cladding, when the metallurgical combination is satisfied, the dilution rate should be reduced as much as possible. Studies have shown that the minimum dilution rate that can be obtained when different substrate materials are alloyed with the layer search is generally considered that It is advisable to keep the dilution rate below 5%.


Stomata. Porosity is also a very harmful defect in the laser cladding layer. It not only easily becomes a source of cracks in the cladding layer, but also greatly harms the cladding layer that requires high air tightness. In addition, it will also directly affect Wear resistance and corrosion resistance of the cladding layer. The main reason for this is that the coating powder is oxidized, wet or some elements undergo oxidation reaction at high temperature before laser cladding, and gas will be generated during cladding. Furthermore, the sub-laser treatment is a rapid melting and solidification process. If the generated gas is not discharged in time, it will form pores in the coating. In addition, there are multiple overlapping holes in the overlap cladding, solidification holes brought about by the solidification and shrinkage of the cladding layer, and bubbles caused by the swallowing of certain substances during the cladding process.


Generally speaking, the pores in the laser cladding layer are unavoidable, but compared with the thermal spray coating, the pores in the laser cladding layer are significantly reduced. During the laser cladding process, some measures can be taken to control it. The commonly used method is to strictly prevent the oxidation of the alloy powder during storage and transportation. Before use, drying and dehumidification and laser cladding should be taken to prevent oxidation. According to the test, choose Reasonable laser cladding process parameters.


3. The composition and structure are not uniform during laser cladding. In the process of laser cladding, component unevenness is often generated, so-called component segregation and the resulting uneven structure. There are many reasons for component segregation. First, when laser cladding is heated, its heating speed is extremely fast, which will bring about a great temperature gradient in the direction from the substrate to the cladding layer. The existence of this gradient will inevitably cause the orientation of the cladding layer to solidify successively during cooling. According to metallographic knowledge, it is known that the components of the cladding layer that have solidified successively are different. In addition, the cooling speed is extremely fast after solidification, and the element has no time to homogenize the heat diffusion, which leads to the occurrence of non-uniform composition, so-called component segregation. At the same time, it will naturally cause the unevenness of the structure and the damage of the performance of the cladding layer. This component segregation cannot be resolved in laser cladding. Second, component segregation caused by convection in the molten pool. Due to the uneven distribution of laser radiation energy, convection in the molten pool must be caused during cladding. This convection in the molten pool often causes macroscopic homogenization of alloying elements in the coating, because the material transfer in the melt mainly depends on liquid flow (that is, convection). To achieve, at the same time, the convection of the molten pool will also bring about the segregation of the composition. In addition, due to the properties of the alloy, such as viscosity, surface tension, and interactions between alloy elements, they will affect the convection of the molten pool, so they will also affect the segregation of the composition. It is impossible to completely eliminate component segregation in laser cladding. However, by adjusting the interaction time between the laser and the cladding metal or adjusting the laser beam type to change the overall convection of the molten pool to multi-convection convection and other process parameters to change the process parameters to properly suppress the segregation of the composition of the laser cladding layer, in order to obtain a more structure Uniform cladding layer to meet the designed coating performance. During the multi-lap cladding, due to the cooling rate of the lap zone and the heterogeneous crystal nucleation at the lap zone, the lap zone has a different structure from the non-lap zone, thus making the multi-lap laser cladding The tissue in the cover is uneven.


Fourth, cracking and cracking. Since the birth of laser cladding technology, in general, it has not been able to truly promote its application. This is mainly because the most difficult problem in laser cladding is the cracking and cracking of the cladding layer, which greatly limits the application range of this technology. The main cause of laser cladding cracks is due to the difference in physical properties between the laser cladding material and the substrate material, coupled with the rapid heating and rapid cooling of the high-energy density laser beam, which causes extremely thermal stress in the cladding layer. Generally, the thermal stress of the laser cladding layer is tensile stress. When the local tensile stress exceeds the strength limit of the coating material, cracks will occur. The dendrite boundaries, pores, and inclusions of the laser cladding layer have low strength. And Yizi produces stress concentration, and cracks often occur in these places. In terms of laser cladding materials, low melting alloy materials can be added to the cladding layer, which can alleviate the stress concentration in the coating and reduce the tendency to crack. Attempting to add a suitable pot of rare earth to the laser cladding layer can increase the toughness of the coating and significantly reduce the crack of the cladding layer during the laser cladding process. Although these measures can solve some problems, they still cannot solve the cracks, pores and inclusions of titanium alloy cladding, so it is necessary to develop materials suitable for titanium alloy cladding. In terms of the laser cladding process, in order to obtain a high-quality cladding layer, a new type of laser cladding technology can be further developed. For example, gradient coating uses a method of gradually changing the hard phase content to obtain a continuous hard phase content in the cladding layer. Varying and crack-free gradient melting layer. In addition, proper heat treatment before and after the coating, such as preheating and laser remelting, can also effectively prevent cracks and holes in the cladding layer.


基础研究与实践的民营企业,更多详情可访问我们的网页:http://xczlsj.com。 Dongguan Yingchuang Laser Technology Co., Ltd. is a private enterprise specializing in the basic research and practice of laser welding machines and laser cladding equipment . For more details, please visit our website: http://xczlsj.com.

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